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Objects & matter opaque to visible light, transparent to high & low frequencies. Why?

  • 12-05-2020 1:44pm
    #1
    TheBoyConor
    Closed Accounts Posts: 3,292 ✭✭✭


    I have been wondering about this but I have not been able to find a clear answer.

    Everyday matter and and the vast majority of objects and materials we see around us every day, they are nearly all completely opaque to visible light and adjoining parts of the spectrum UV and IR. Yet the same materials and objects are largely transparent to the higher frequencies of electromagnetic radiation like X-rays and gamma rays and also the lower frequency radio waves and, to some extent, microwaves.
    The obvious exceptions are things like glass, water, and some other liquids. I am supposing gasses are mainly transparent owing to their very low density. Indeed, most of the gasses in air are also transparent in the liquid phase.

    Obviously, this property of matter makes the sense of vision possible. If visible light were to pass through matter then it would mean the sense of vision would not be of anywhere near the same level of use as you could see very little - it would appear as if everything were made of clear glass.

    Why is this? What is it about matter that makes it opaque to some wavelengths of electromagnetic radiation, such as visible light and it's closest neighbours on the spectrum, yet makes it transparent to much shorter and much longer wavelengths?


Comments

  • #2
    Michael Collins
    Moderators, Science, Health & Environment Moderators Posts: 1,846 Mod ✭✭✭✭Michael Collins


    It pretty much all depends on whether the photons incident on the material are absorbed or not. If you consider a single atom, a photon will be absorbed if it can promote an electron up to a higher energy level. This happens at discrete frequencies i.e. the frequencies corresponding to the energy difference between the orbitals. So in a gas, where each atom is separate, only photons at certain frequencies will be absorbed (see line spectrum).

    Now, when you bring a group of molecules together to form a material, the wavefunctions of each molecules overlap. By the Pauli exclusion principle, none of the electrons can occupy the same state, so you get a splitting of the discrete energy levels into what are effectively continuous bands (since there are so many molecules). This means most solids can absorb many different frequencies.

    In some materials, however, the energy levels split into two separated ranges, known as the valence band and the conduction band. The gap between them is known as the band gap. If the difference in energies between the two bands is greater than the energy corresponding to photons in the visible range, the material will be transparent -- since there are no available energy levels for an electron inside the band gap, a photon cannot be absorbed and instead passes through.

    For example, searching online I see that a specific type of glass has been measured to have a band gap of 2.56 eV. This means no photons can be absorbed that have energy less than this, since there is nowhere for the valence band electrons to go.

    More quantitatively:

    For E < 2.56 eV the glass will be transparent.

    But Plank's relation gives E = h.f, so the glass will be transparent for

    h.f < 2.56 eV

    i.e. for frequencies

    f < E/h = (2.56 eV)/(6.626×10^−34 J.s) = (4.1016×10^−19 J)/(6.626×10^−34 J.s) = 619 THz

    Visible light ranges from about 400 THz (red) to 700 THz (blue and violet), meaning most light will get through, apart from maybe some in the higher end of the violet spectrum.

    This also means you probably cannot get sunburnt through this particular kind of glass!


  • #3
    TheBoyConor
    Closed Accounts Posts: 3,292 ✭✭✭TheBoyConor


    That explains it fairly well for me. Thanks.

    So you know the way lead is often used for shielding against gamma rays. Is lead's effectiveness in shielding against the gamma rays a result of the difference in it's bad gap corresponding to the photon energy of gamma rays? Or is it more on attributable to the fact that lead is just basically denser and , kg for kg, it is no more effective at shielding that any other material?

    I assume that when photons are absorbed, their energy is just converted into heat in the material? Right? Obviously this happens with IR and microwaves as we see examples of this every day. But is it the same with all wavelengths?

    You explain absorption well there but what is the difference between that and objects opaque due to reflecting light rather than absorbing? Is reflectance a result of the band gap or is that caused by some other mechanism?

    Which also brings me to another question - obviously visible light gets bent and refracted by lenses - for the likes of gamma rays and radio waves, will a convex lens focus those wavelengths in the same fashion as with visible light? Obviously the material that the lense is made from will need to be transparent to those wavelengths as you describe. Since "c" for all wavelengths is the same so I am imagining that a convex shaped piece of concrete could, in theory refract and focus a beam of gamma or radio waves. Why is it then that radio wave guides are always hollow structures that rely on reflection and constructive/destructive interference of the waves to focus a beam rather than using a lens and refraction technique?


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